1. Field of the Invention
This invention relates to an improved process for molding polyisocyanate-based polymers and is more particularly concerned with improving the mold release properties of said polymers through the use of internal release agents.
2. Description of the Prior Art
The molding of polyisocyanate-based articles both cellular and non-cellular, in a variety of shapes and using an assortment of polymer compositions and mold materials, is well known in the art. To prevent the molded pieces from sticking to the mold surfaces, mold release agents are generally employed. These agents can be in the form of external or internal release agents. The external agents, substances such as waxes, soaps, oils, and the like, are applied at regular intervals to the mold surface. The molding operation must be stopped frequently to allow for cleaning and reapplication of fresh release agent. The internal type of agents are much the preferred release agents because they invariably provide for multiple releases before the mold must be closed down for cleaning. This, in turn, leads to the production of a greater number of parts in a given time period for a mold using internal as opposed to external release agents.
Generally speaking, the internal release agents are derived from polysiloxanes which can be divided into two broad classes; (1) release agents that have terminal silicon to carbon bonds and are thereby stable to hydrolysis (non-hydrolyzable); and (2) release agents that have terminal silicon to oxygen to carbon bonds and are thereby susceptible to hydrolysis (hydrolyzable). In most cases the non-hydrolyzable release agents are preferred because of their greater stability, particularly in the basic reactive polyurethane forming environment.
Typical examples of the use of the non-hydrolyzable type of release agents can be found in German patent application 23 634 52 which discloses the esterification products derived from mono- or polycarboxylic acids (8 to 40 carbons) with polysiloxanes having hydroxymethyl groups attached directly to silicon atoms; U.S. Pat. No. 3,993,606 which discloses the salts of fatty acids with certain amino-polysiloxanes wherein the amino groups are connected to the silicon through a methylene radical; U.S. Pat. No. 4,220,727 which discloses, inter alia, certain organofunctional carboxyalkylsiloxanes; U.S. Pat. No. 4,350,777 which discloses various hydroxyl, primary amino, and secondary amino terminated polysiloxanes; U.S. Pat. No. 4,033,912 which discloses the reaction products of mono- or polyisocyanates with polysiloxanes having hydroxymethyl, or certain secondary aminomethyl, groups attached to the silicon atoms; and German patent application No. 30 121 26 which discloses hydroxyl terminated reaction products derived from the reaction of an excess of at least difunctional alcohols or aminoalcohols with isocyanate terminated prepolymers which latter are derived from the reaction of an excess of a polyisocyanate with a hydroxy, amino, or mercapto terminated polysiloxane of the non-hydrolyzable type.
Typical examples of the use of the hydrolyzable type of polysiloxanes can be found in U.S. Pat. No. 4,038,221 which discloses tertiary aminoalkoxy polysiloxanes not as internal mold release agents per se but as a replacement for various emulsifiers, stabilizers, and activators, in the preparation of cellular polyurethanes; U.S. Pat. No. 4,052,495 which discloses a class of hydroxyl terminated polyoxyalkyleneoxy siloxanes; and British Pat. No. 1,494,930 which discloses salts of amino group containing polysiloxanes with a fatty acid having more than 7 carbon atoms wherein the amino group containing polysiloxane substrates include primary, secondary, and tertiary amino terminated alkylene (non-hydrolyzable) and alkyleneoxy (hydrolyzable) siloxanes.
Those release agents which carry organofunctional end groups such as hydroxyl and amino have generally been employed in some prereacted form rather than directly in a one-shot process. This is done to avoid stability problems in the event the polysiloxane release agent is of the hydrolyzable type, and because hydroxyl terminated polysiloxanes are notoriously incompatible and unreactive with polyurethane forming ingredients. For example, U.S. Pat. Nos. 4,033,912 and 4,350,777, cited supra, specifically in the former and preferably in the latter, call for the use of the release agent in prepolymer form to be added to the urethane forming ingredients in the mold. In U.S. Pat. No. 4,052,495 the reactive hydroxyalkyleneoxysiloxanes are added to an uncured non-cellular urethane prior to the curing of the latter in a mold. For the sluggishly reactive types of internal release agents, such prereactions described above are particularly necessary where a rapid molding procedure is called for, as, for example, in RIM preparations.
We have now discovered an improved process for the production of molded polyisocyanate based polymers wherein the use of a particular class of primary or secondary amine terminated hydrolyzable type polysiloxane imparts outstanding release properties to the molded articles. In fact, the exceptionally large number of mold releases which can be obtained before the mold must be refurbished makes the use of the present process particularly appropriate in RIM applications.
Additionally, the molds containing the polymers prepared in accordance with the present invention can be opened much more easily than those molds containing formulations employing prior art release agents.
In yet another surprising feature, molded polyurethane articles in accordance with the present invention possess a high degree of paintability. The majority of the prior art release agents provide parts which do not have good paintability.
In contrast to the prior art hydroxyl terminated polysiloxane release agents of the type noted above, the present compounds enjoy a wide range of compatibility with the polyurethane forming ingredients. Additionally, they react faster into the polyurethane matrix than the hydroxyl polysiloxanes. These attributes thereby eliminate the prior art necessity of prereacting the agents with either polyisocyanate or polyurethane prepolymer and the polyurethane forming process can be carried out in a one-shot type process if desired. At the same time, the release agents are locked into the formulation and cannot migrate from the molded part at some later time.
Surprisingly, it has been discovered that the primary amino-polysiloxanes used in accordance with the present invention, and, which correspond to those primary aminoalkyleneoxypolysiloxanes employed in the British Pat. No. 1,494,930 in the form of the acid salts, behave quite differently from the reference release agents.
A comparison of the reactivity of polyurethane formulations employing the internal mold release agents in accordance with the present invention as opposed to formulations in accordance with the British reference discloses the much faster reactivities of the former over the latter. In fact, the slowness of the prior art formulations, in some cases, would preclude their use in preparing RIM parts, particularly large RIM parts.